The invention relates generally to motion sensors and more particularly to motion sensors based on ultra-wideband radar.
Motion sensors are primarily based on ultrasound, passive infrared (PIR) and radar detectors. Ultrasonic motion sensors are commonly used for automatic door openers and security alarms. They are of low cost and can operate with narrow beamwidths. However, installation options are limited since ultrasonic beams are easily blocked by thin materials, including paper. Another short-coming is the tendency to false trigger on reflections from blowing curtains or flying insects.
PIR sensors are perhaps the most frequently used home security sensor. They use a special Fresnel lens to generate multiple thermal images of a warm object, such as a person. As the person traverses the field of view, the thermal images produce periodic fluctuations as they sweep across an IR sensor element, which are then detected by low cost electronics. As with ultrasound, PIR can be blocked by a sheet of paper. Furthermore, PIR has no range adjustment.
FM-CW radar sensors emit a swept frequency microwave signal and compare the frequency of the echo with its current emission frequency, producing a beat frequency proportional to range. FM-CW radar sensors use low cost microwave oscillators and detector diodes, and audio frequency processing electronics to determine the audio beat frequency, or range. Nonranging radar sensors simply emit a continuous microwave carrier and listen for a Doppler shift from a moving object. The antennas are usually low cost, die cast metal horns. The main limitations to FM-CW and Doppler radar are:
1) limited materials penetration due to the high microwave frequencies employed, PA1 2) microphonics caused by the use of short radar wavelengths combined with audio frequency processing, PA1 3) frequency crowding, and PA1 4) poor short-range operation. Short range performance is limited by close-in noise sidebands in the transmit oscillator, which must operate in the gigahertz region and yet not have random frequency variations on the order of 100 Hz, since this variation would be indistinguishable from the desired beat frequencies.